1. Field of the Invention
The present invention relates to multi-layer pigments, as well as to inkjet ink compositions comprising them.
2. Description of the Related Art
An inkjet ink composition generally consists of a vehicle, which functions as a carrier, and a colorant such as a dye or pigment. Additives and/or cosolvents can also be incorporated in order to adjust the inkjet ink to attain the desired overall performance properties.
In general, pigments alone are not readily dispersible in liquid vehicles. A variety of techniques have been developed which can provide stable pigment dispersions that can be used in inkjet printing. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants include water-soluble polymers and surfactants. Typically, these polymeric dispersants have a molecular weight less than 20,000 in order to maintain solubility and therefore pigment stability.
The surface of pigments contain a variety of different functional groups, and the types of groups present depend on the specific class of pigment. Several methods have been developed for grafting materials and, in particular, polymers to the surface of these pigments. For example, it has been shown that polymers can be attached to carbon blacks containing surface groups such as phenols and carboxyl groups. However, methods which rely on the inherent functionality of a pigment's surface cannot be applied generally because not all pigments have the same specific functional groups.
Multiple layers of charged materials have also been used to modify particle properties. For example, U.S. Pat. No. 6,479,146 and U.S. Patent Publication No. 2002/0187197 describe a process for coating particles with consecutive alternating layers of oppositely charged polyelectrolytes. These are then further treated to remove the inner core. Multi-layering processes are also described in EP 1 190 123, U.S. Patent Publication No. 2004/0013721, and U.S. Pat. No. 6,203,909. However, none of these methods has been used for the preparation of pigments for inkjet ink applications.
Methods for the preparation of modified pigment products have also been developed which can provide a pigment with a variety of different attached functional groups. For example, U.S. Pat. No. 5,851,280 discloses methods for the attachment of organic groups onto pigments including, for example, attachment via a diazonium reaction wherein the organic group is part of the diazonium salt.
Other methods to prepare modified pigments have also been described. For example, PCT Publication No. WO 01/51566 discloses methods of making a modified pigment by reacting a first chemical group and a second chemical group to form a pigment having attached a third chemical group. Ink compositions, including inkjet inks, containing these pigments are also described. Also, U.S. Pat. No. 5,698,016 discloses a composition comprising an amphiphilic ion and a modified carbon product comprising carbon having attached at least one organic group. The organic group has a charge opposite to the amphiphilic ion. Also disclosed are aqueous and non-aqueous ink and coating compositions incorporating this composition, including ink jet ink compositions.
While these methods provide modified pigments having attached groups, there remains a need for improved processes for attaching groups and, in particular, polymeric groups, to a pigment. These additional methods may provide advantageous alternatives to forming modified pigments.